Gas chromatography is commonly used in analytical chemistry for separating and analyzing compounds of a sample. For example, a gas chromatograph may be used to test the purity of a sample, identify a compound, separate different components of a mixture or to prepare (e.g., purify) compounds from a mixture. Gas chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas are absorbed and desorbed by a stationary phase material in a column. A plug of the sample is injected into a steady flow of carrier gas. Interactions between this stationary phase material and the various components of the sample—which differ based upon differences among partition coefficients of the components—cause the sample to be separated into the respective components. At the end of the column the individual components are more or less separated in time. Detection of the vapors provides a time-scaled pattern which, by calibration or comparison with known samples, indicates the constituents and their concentrations in the test sample.
Typically, the main components of such a system are the column, an injector for introducing the sample into carrier gas and passing the mixture into the column, a device for transferring sample into the injector, a detector at the outer end of the column, gas controls, and a device such as a computer for processing and displaying the output of the detector. An oven may be used to elevate temperature to maintain the sample in a volatile state, and to improve the discrimination of constituents. The injector may include a resealable septum through which a needle can be inserted to inject a sample into the injector.
It is also known to provide gastight, resealable septa on vials used to contain samples.